The expression “optical information” is used in this description and in the following claims to indicate any graphic representation having the function of storing a piece of information, whether coded or uncoded. A particular example of optical information is given by linear or two-dimensional optical codes, in which the information is coded by means of appropriate combinations of elements of preset form, for examples squares, rectangles or hexagons, of a dark colour, (usually black) separated by light elements (spaces, usually white), such as bar codes, stacked codes and two-dimensional codes in general, colour codes, etc. More in general the term “optical information” further comprises other graphic forms, including printed characters (letters, numbers, etc.) and particular patterns (such as stamps, logotypes, signatures, fingerprints etc.). The term “optical information” comprises graphic representations detectable across the whole range of wavelengths, between infrared and ultraviolet, and thus not limited to the field of visible light.
In the present description and in the following claims the expression “apparatus for image acquisition” means a fixed or portable apparatus capable of acquiring images of objects or people, of subjects in general, and in particular of optical information, by means of a plurality of possible acquisition techniques.
For example, acquisition can take place by illuminating a subject, collecting with a suitable optical receiving system the light diffused by the object on a sensor consisting of an array of light-sensitive elements of a linear or matrix type, for example of the CCD or C-MOS type, and generating an image signal by means of an integrated or separate electronic system associated to the sensor. The image signal, generated in analog or digital form; can then be processed by the same apparatus or in a separate image processing system.
Typically, in apparatuses for reading coded optical information, such as bar codes, the image signal in digital form is decoded to extract the content information of the code.
Apparatuses of this type are known as linear or matrix TV cameras or cameras, and when they read optical information, they are also known as code-reading devices of the “imager” type.
According to another technique, the acquisition can be performed by illuminating a subject by scanning with one or more laser beams, by collecting the light diffused or reflected by the subject upon one or more photodiodes using an optical receiving system, and by generating, using a dedicated electronics, an electric image signal representing the diffusion/reflection of each point of the object that was struck by the laser beam during scanning. This signal is then processed and, in particular for apparatuses that read coded optical information, is digitalised and decoded. Apparatuses of this type are generally known as “laser scanners”. Instead of capturing an image with a single simultaneous acquisition of all the light-sensitive elements (“in parallel”), as in TV cameras and “imager” reading devices, “laser scanners” capture the image of the object sequentially during the scanning process, one instant after another (“in series”). Optical code readers, for example two-dimensional optical codes (Datamatrix, QR, PDF, Maxicode, etc.), in particular readers of the “imager” type with matrix sensors, are provided with aiming devices which as accurately as possible indicate to a user the Field of View (FoV) of the reader, that is, the framed area as the distance between the reader and the optical code varies. In particular, it is preferable that the area aimed by the aiming device (aiming area) is always included within the area actually framed. In this way, the user can be certain that if the optical code to be read is situated within the aiming area, it is indeed framed by the reader.
Readers of the “laser scanner” type, especially those that use lasers with non-visible wavelengths, can also comprise aiming devices capable of indicating to the user the zone where scanning is performed.
EP 0997760 describes an aiming device for an optical information reader comprising two lighting units, each of which comprising a light source, for example a LED, or a lamp, and a V-shaped light guide, arranged downstream of the respective light source in order to generate a pair of optical emission paths. Each V-shaped light guide comprises a pair of branches arranged at a predetermined angle. Furthermore, the light sources can be suitably inclined relative to a reading plane containing the optical information.
As a result, the above-described aiming device generates four light beams defining on the reading plane vertices of a quadrilateral, which provides the user with a visual indication of a reading zone framed by the optical information reader.
A drawback of the above-described aiming device is that a part of the light beam entering the V-shaped light guide of the aiming device escapes from it through the portion located where the light guide branches intersect.
This part of the light beam produces an indefinite image on the reading plane, which image is thus added to the reference images produced by further portions of light beam that have traversed the branches of the V-shaped light guide and exit from these branches. In other words, the above-mentioned part of luminous beam creates a disturbance which not only prevents accurate indication of the reading zone framed by the optical information reader, but it can even cause confusion relative to the extension and position of the reading zone for an operator using the optical information reader.
Only the light rays emitted by the light source, which have a substantially parallel direction to one of the branches of the V-shaped light guide contribute to forming the above-mentioned further portions of light ray, and consequently the reference images on the reading plane. All the other light rays emitted by the light source follow an uncontrolled path within the light guide and can leave the light guide in equally uncontrolled directions, forming the indefinite image referred to above. In addition, the quantity of these rays that are substantially parallel to one of the branches of the guide is very small, or even negligible, compared to the total number of rays emitted by the source, so that it is difficult for an operator to identify visually the above-mentioned reference images on the reading plane.